1. Field of the Invention
This invention relates to storage stable thermofusible, thermosetting, adhesive systems formed by the reaction of a polyisocyanate with a polymer with functional groups that are reactive with the isocyanate group. More particularly, the invention relates to storage stable thermofusible, adhesive paste formulations which can be used for bonding various materials particularly textiles.
2. Description of Related Art
Thermoplastic adhesive formulations for the purpose of bonding textiles generally have failed either during washing or during dry cleaning. Thermosetting formulations generally have failed during storage prior to application or after coating and fixing but before lamination. Such formulations have included systems which are composed of one or two components. In certain two component systems, one component is a crosslinking agent, and the second is a polymer with functional groups capable of reacting with the crosslinking agent. These two react in what is called a cure. When cured the material desirably has irreversibly become a crosslinked adhesive.
A crosslinking agent that contains isocyanate groups and a polymer with isocyanate reactive groups is an example of a two component system, and the one of interest here. In this case, the isocyanate groups can be chemically deactivated, allowing the isocyanate crosslinking agent to be stored in the same medium as the isocyanate reactive polymer without reaction. The application of heat re-activates the isocyanate crosslinking agent, and allows the thermosetting reaction to begin.
To be used for textile bonding, thermosetting adhesives must meet a number of very stringent processing conditions, and must also be capable of use in a very demanding environment once cured.
Typically, the processing of textiles requires application of the adhesive formulation to the textile in advance of bonding and curing. The textiles with the non-cured adhesive already applied are then stored, possibly for months. The formulation must then be storage stable under a range of temperature and humidity conditions. This means that the thermosetting reaction must not initiate before it is intended.
Once the adhesive bond is formed and the thermosetting reaction (curing) is completed, the adhesive must be able to withstand a wide variety of conditions during use. This includes resistance to large volumes of hot water and detergents during washing, and resistance to dry cleaning chemicals.
In certain textile applications, the formulation must also be capable of being coated and fixed on to a textile material and, finally, laminated to another textile material without excessively penetrating into the textile material. This means that there are specific viscosity requirements on the system. However, a system that is highly viscous typically will achieve a lower degree of crosslinking. A system with a low degree of crosslinking will have a relatively poor resistance to heat, steam, solvents etc. On the other hand, a system that is too low in viscosity can excessively penetrate into the fabric and cause undesirable marks on the fabric (strike-through and strike-back) and can provide low bonding strength between fabric layers.
There is a need, therefore, for adhesive systems that have a high enough viscosity to avoid excessive penetration into the fabric, but that allow the reaction to achieve a high degree of crosslinking and, thereby, form a strong adhesive bond with good resistance to heat, solvents, steam etc.
U.S. Pat. No. 4,888,124 to Blum et al., describes a general method of preparing heat-crosslinkable adhesive systems that can be stored as low viscosity aqueous dispersions. Adhesives prepared by this method are a mixture of two primary components. One component is polyisocyanate particles that have been deactivated and are storage stable. Deactivation typically involves reacting a liquid dispersion of polyisocyanate particles with a bi-or polyfunctional reactant. The bi-or polyfunctional reactant is added to the polyisocyanate in less than the stoichiometric amount, and reacts only at the surfaces of the polyisocyanate particles in the dispersion. This forms a protective shell around each polyisocyanate particle.
The protected polyisocyanate particles can then be stored at room temperature in a low viscosity dispersion that contains a second component, i.e., a polyisocyanate reactive polymer, for long periods of time without further reaction. When it is desired to begin the thermosetting reaction to form the adhesive, heat is applied. At high enough temperatures the protective shell is removed from the polyisocyanate particles and the thermosetting, adhesive reaction between the polyisocyanate particles and the isocyanate reactive polymer will then proceed. However, these aqueous dispersions as described are not desirable for use in connection with textile applications.
U.S. Pat. No. 6,348,548 to Abend describes adhesives formed from storage stable polyisocyanate dispersions with aqueous dispersions or solutions of isocyanate reactive polymers, and that are used as coverings in the form of dried films. The polyisocyanate particles are encased in a polymer protective shell. These storage stable, solid films can be formed on a substrate without starting the thermosetting, adhesive reaction. Again, once it is desired to begin the thermosetting, adhesive reaction, heat is applied. The application of heat removes the protective shell and allows the reaction to begin. However the systems used have insufficient storage stability at ambient temperature to be desirable for use in connection with textile applications.
U.S. Pat. No. 4,849,262 to Uhl et al., describes printing pastes and dyeing liquors containing finely divided, solid dispersions of polyisocyanates mixed with binders. The polyisocyanates are present in the system at from 0.1 to 6.0% by weight and the binders are present in about 2.4% to 12% by weight. The binders contain sufficient polar groups to allow them to be dispersed in water without assistance. The fixing process requires a heat treatment of 100° C. for about 3–6 minutes.
U.S. Pat. No. 3,922,418 to Lauchenauer describes crosslinkable resin systems in which the crosslinking agent is in contact with the resin particles only at their surfaces. The amount of crosslinking agent in the system is insufficient to crosslink more than about 40% of the resin. In addition, the Lauchenauer crosslinking agent is “masked,” meaning that all the reactive groups of the crosslinker are chemically deactivated and not just those on the surface of the particles. Thus, the curing process tends to be slow and stops when heat is removed.